Global environmental issues, such as global warming and ozone depletion, have recently been coming to the front. CO2 emission, in particular, which is said to have a significant impact on global warming, is a considerable concern, and its regulation standards are attracting interest in each country.
One of the major challenges in CO2 reduction is to reduce energy loss caused by friction loss in machinery, systems, and the like, in particular, to reduce vehicle fuel consumption. For reducing friction of parts having relatively movable, facing contact surfaces in engines and the like, such as sliding surfaces, rotating surfaces, or rolling surfaces, an important role is played by materials forming such contact surfaces, and lubricants for lubricating such contact surfaces adapted to each material.
The material forming the contact surfaces is required to give an excellent anti-wear property and a low frictional coefficient to the parts in engines or the like under severe frictional wearing. For these purposes, various hard thin film materials have recently been employed. For example, a DLC material is expected as a low friction material for its lower frictional coefficient in the air in the absence of a lubricant, compared to an anti-wearing hard coating material, such as TiN and CrN.
For reducing energy loss in lubricants, for example, for improving engine fuel consumption, there have been proposed to reduce viscous resistance in hydrodynamic lubrication areas and agitation resistance in engines by lowering the viscosity of lubricants, and to reduce frictional losses in mixed and boundary lubrication areas by adding optimum friction modifiers and various additives. The friction modifiers have widely been researched, in particular, organic molybdenum compounds, such as molybdenum dithiocarbamate (MoDTC) and molybdenum dithiophosphate (MoDTP), and lubricants containing organic molybdenum compounds have been developed and achieving effects, which exhibit an excellently low frictional coefficient on conventional steel sliding surfaces in the initial stage of use.
On the other hand, it has been reported that DLC materials, which have an excellent low friction property in the air, can offer only limited friction reducing effect in the presence of a lubricant (Non-patent Publication 1). It has also been reported that application of a lubricant containing an organic molybdenum compound to DLC materials does not result in sufficient friction reducing effect (Non-patent Publication 2).
Non-patent Publication 1: Japanese Society of Tribologists, Congress Proceeding, Tokyo, 1999.5, p11-12, Kano et al. Non-patent Publication 2: World Tribology Congress 2001.9, Vienna, Proceeding p 342, Kano et al.